Amplifier for underwater listening apparatus



Nov. 8, 1949 Filed Jan. 5, 1946 F. P. HERRNFELD AMPL IFIER FOR UNDERWATER LISTENING APPARATUS 3 Sheets-Sheet l HYDROPHONE ?RVEAMPLIFIVER FILTER NETWORK I. BASS -BOOST 2. FLAT 3. SOO- U-HIGH PASS 4. 3000-Pd- HIGH PASS 5. GOOO-N-HIGH PASS WMAGNETIZER AMPLIFIER HEADPHONES LOUDSPEAKER SUPERSONIC DETECTOR ELECTRON RAY INDICATOR FIG. I

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gwuz/wkw FRANK P HERRNFELD Patented Nov. 8, 1949 AMPLIFIER FOR UNDERWATER LISTENING APPARATUS Frank P. Herrnfeld, Los Angeles, Calif., assignor to the United States of America as represented by the Secretary of the Navy Application January 5, 1946, Serial No. 639,456

1 Claim.

This invention relates in general to audio amplifiers and more particularly to audio ampIifiers used with listening gear for submarines and surface craft.

In general, the amplifier of this invention is operated with a signal from a magnetostriction hydrophone and. the amplifier output is supplied to a pair of headphones. The hydrophone is mounted to be oriented through an angle of 360 by remote means which are indicative of the direction of the hydrophone. When used with a baffle the hydrophone has very directional characteristics. Hence, the operator may turn the hydrophone to a point where the signal is a maximum in the headphones, and the resulting bearing of the hydrophone will be the bearing of the source of signals.

If the listening gear is mounted on a surface ship, it may be used for the detection of submarines, either submerged or afloat, or the approach of other surface ships at night or in a fog. The listening gear is usually mounted on submarines for the same purpose, and it becomes a very important piece of apparatus when the submarine is submerged, serving more or less as the eyes of the submarine.

Various problems arise in the development of the apparatus of this invention, for instance certain underwater noises are known to have certain frequencies, and for this reason a multiband filter is required in the amplifier.

oftentimes it is desirable to know if the signals contain supersonic frequencies. In order to obtain this information a supersonic detector must be included in the amplifier.

When magnetostrictive hydrophones are used requiring magnetizing at certain intervals, a magnetizer is included in the amplifier.

To supplement the head phones, provision is usually made for a loudspeaker. In certain cases where the maximum signal output is critical, such as in bearing determination, an electron-ray indicator or a vacuum-tube voltmeter is included in the amplifier circuit.

This invention consists in providing an amplifier capable of producing optimum operation of standard listening gear apparatus. Accordingly an object of this invention is to provide an amplifier capable of receiving underwater signals from a hydrophone, amplifying and treating said signals until they become intelligible to the operator.

Another object is to provide an amplifier that indicates the signal audibly by headphone and loudspeaker, and visually by an electron ray or vacuum tube voltmeter indicating device.

Another object is to provide an amplifier that is capable of magnetizing the hydrophone at intervals where such magnetization is required by the hydrophone.

Another object is to provide an amplifier that is capable of selectively indicating the presence of supersonic frequencies.

Another object is'the provision of an amplifier system which will be economical to manufacture, reliable in operation and which possesses all of the qualities of ruggedness and dependability in service.

Figure 1 is a block diagram of the apparatus of this invention.

Figure 2 is a detail circuit diagram of the amplifier of this invention.

Referring to Figure l, the hydrophone I may be of any type, though for the purposes of this invention a magnetostrictive hydrophone is preferred. The structure of the hydrophone is such that the responsive unit has the shape of a long cylinder, which when semi-encased in a bafiie has a marked directional response characteristic. The hydrophone may be constructed in two sections so that the windings may be series connected either aiding or opposing.

Preamplifier 2 may be a simple three-stage amplifier; or a two-stage, resistance-coupled amplifier; or a three-stage, resistance coupled, negative feedback stabilized amplifier. The latter is the-preferred form.

The filter network 3 may have the following frequency bands, l-fiat, 2-500 cycle high pass, 3-1500 cycle high pass, 4-3000 cycle high pass, and 5-5500 cycle high pass.

It may have the following combination, l-flat, 2-600 cycle high pass. 3-1200 cycle high pass, and 4-2400 cycle high pass.

It also may have the following combination, 1-bass boost, Z-fiat, 3-500 cycle high pass, 4-3000 cycle high pass, and 5-6000 cycle high pass.

It also may have the following combination, l-bass boost, 2-2000 cycle low pass, B-fiat, 4-500 cycle high pass, 5-3000 cycle high pass, and 6-6000 cycle high pass.

Or it may have the following combination, l-bass boost, 2-2000 cycle low pass, 3-fiat, 4-500 cycle high pass, 5-1500 cycle high pass, and 6-4500 cycle high pass.

It is obvious that various combinations of bands may be used. While they cover approximately the same bands, the exact figures may vary some what due to the particular design of the remainder of the apparatus.

The final amplifier may be a single tube, or it may be a three-stage amplifier for the operation of a loudspeaker. In a preferred form it is a twostage amplifier with one tube working singly and two tubes workingas a push-pull stage in the output. The conversion from single to push-pull stage is accomplished by phase inversion. A small fraction of the output voltage is fed back to the cathode of the first stage of thetwo-stageamplifier to reduce harmonics and to' stabilize the amplifier against variationsin plate supply voltage. A diode may be connected, through a switch, to the control grid of one of the pushpull tubes. It is used as a rectifier, making it possible to detect supersonic signals which would otherwise go unnoticed by the listener. Thistwostage amplifier may also be made into a threestage amplifier by adding another tube working singly, and the tubes may be resistance-capacity coupled. The preamplifier and the final amplifier are designed for either D. C. or A. C. operation, the. choice depending upon the type of power supply available.

The output of the final amplifier may operate one or more indicating devices. It has already been pointed out that headphones 5 and/or 1oudspeakers 5 may be used as indicators and that supersonic detection 8 is added to the amplifier to render it more useful for the purpose specified.

Anotherv type of indicator frequently used is the electron-ray indicator i and/ or the electron voltmeter. These indicators are generally utilized when the apparatus is used for hearing determination, because a more accurate indication of themaximum signal is possible with these. indicators than with those of the aural type. The electron ray indicator is operated from the output of a high pass filter for greater accuracy at high frequencies.

When the magnetostrictive hydrophone isused, it is desirable to send a magnetizing current through it at intervals for magnetization purposes. This is accomplished by switching condensers, that are maintained charged from the output of the final amplifier, to discharge through the hydrophone. This discharge of the condensers. through the hydrophone remagnetizes the transducer as desired. The condensers also have a desirable filtering effect on the final amplifier, that is the reason they are charged from the output circuit. The switch and condensers are generally designated the magnetizer 9 in Figure 1..

The amplifier circuit disclosed in Figure 2 was designed for use with the topside-mounted listening gear for submarines and the through-thehull-mounted listening equipment for surface ships. In general, this amplifier is operated with a signal from a straight magnetostriction hydrophone and its output is supplied to a pair of headphones. Because of its use with the above mentioned hydrophone, facilities are provided to re-.

magnetize the hydrophone by discharging a bank of condensers through it. This amplifier is capable of handling signals as low in intensity as the resistance noise on the input. The amplifier features negative feedback and a simple, but efiective, selective filter circuit having five positions and. employing. only one-coil. The gain is automatically increased asthe filters are inserted in.

order to keep the intensity of the sound approximately constant. The maximum power gain at 1000 cycles with the filter switch in the flat position is 104 db. An electron-ray indicator tube is provided on the panel as an aid in training the hydrophone and it is operated at all times through a 5500 cycle high-pass filter. This filter is used because the hydrophone has its best directive characteristics at high frequencies.

The amplifier of Figure 2 has five stages of amplification. It has an input transformer T-l0l with a nominal impedance ratio rating of 50 to 100,000 ohms. The input transformer feeds a two-stage, resistance-coupled, negative feedback stabilized. amplifier consisting of vacuum tubes V-Hii' and'Vl 02. This amplifier feeds the audio frequency filter assembly directly.

Thev filter assembly consists of impedance matching and loss adjusting networks followed by appropriate filters. The various networks are connected to the circuit by means of a multipole, multiposition switch S Hi l. There are five switch positions, as follows: (1) bass boost, (2) fiat, (3) 500 cycle high pass, (4) 3000 cycle high pass, and (5) 6000 cycle high pass. Step 1 gives the amplifier a peak gain of 122 db. at about cycles, dropping off above 160 cycles at a rate of approximately 6 db. per octave. Step 2 gives the amplifier a flat frequency response with a maximum gain of db. at 1000 cycles. Step 3 gives the amplifier characteristics approximately inverse of step 1, having a maximum gain of 108.5 db. at 9000 cycles, and dropping off below 1500 cycles at the rate of 5 db. per octave; Step 4 has a maximum gain 114 db. at 8500 cycles with a low frequency cutoff of 18 db. per octave below 3000 cycles. Step 5 has a maximum gain of 119 db. at 8500 cycles with a low frequency cut off at 6000 cycles.

The filter is terminated in a continuously variable, tapered volume control consisting of a potentiometer R!5t shunted by a resistor Rl50. This is followed by a three stage amplifier using vacuum tubes Vi 03 and Vl 05 working singly,

and Vl0i and 17-403 working as a push-pullstage in the output. The conversion from single to push-pull stage is accomplished by a phase inversion. A small fraction of the output voltage is fed back to the cathode of the second stage (V-lil5) of the three-stage amplifier to reduce harmonics and to stabilize the amplifier against variations in plate supply voltage.

There are several reasons for using a pushpull stage in the output. Since the amplifier of the particular circuit illustrated in Figure 2 ha to work from a power supply varying from 80 to volts D. C., a voltage amplifier triode or pentode would not give sufficient power output to drive two low impedance type headphones with sufficientvolume. In order to keep the cost at a minimum and to get enough power output fordual headphone operation, the push pull circuit'seemed the best solution. The use of a power pentode, if a 0.3 ampere, 6 volt tube were available, would require an output transformer capable of carrying a D. C. plate current of 15 to 20 ma.

A strapping block in the output gives the user the choice of operating either low or high impedance headphones. When two low impedance phones are used, a small'mismatch for the output tube occurs, increasing the harmonic content slightly when fulloutput from-the amplifier is demanded. It is not often that the full output is required.

The electron ray indicator tube V-l04, preceded by a voltage amplifier V--l09, ahigh pass filter with a 6000 cycle cut off, a half wave rectifier (one half of Vi05) and an RCfilter, is fed from the output of the push. pull power stage through a volume control R--l57. As most of the energy received by the amplifier lies below 6000 cycles, a stage of voltage amplification had to be added to get sufficient sensitivity to operate the electron ray indicator without excessive loudness in the headphones.

The other diode of V-I 05 is connected through a switch to the control grid of one of the push pull tubes and is used as a rectifier, thus making it possible to detect supersonic signals which would otherwise go unnoticed by the listener. The connection of VI05 to the voltage supply through C-I l9 and R-l3|, and by way of switch S-I02 to the grid of Vl01, provides a selective rectifier for high frequency signals which can pass through CI I9. Since they only pass through V-l05, a rectifier tube, in one direction, the result is to alter the bias on the tube V--l'|, thereby causing the sonic signals impressed on the grid to be changed in intensity as heard at the phones, use being made of the non-linear characteristic of the push-pull tubes. action is ineffective with sonic frequency signals and efiective for frequencies of much higher value. Thus closing switch S-I 02 causes a variation in the signal intensity.

Provision is made for remagnetizing the hydrophone when necessary, and this is accomplished by discharging a bank of condensers through the winding of the hydrophone. The magnetizer is a condenser-discharge device consisting of four 40 mid. electrolytic condensers C|33 to C-I34 in parallel, which discharge their energy through a key K-llll into the hydrophone when its cable is plugged into the magnetizer input jack J-l02. With the magnetizer key in the unoperated position, the condensers serve as part of the filter circuit in the push pull output stage. The filter circuit for the push-pull stage is seen by tracing the power circuit from switch S--|0l to resistance R-MB, to normally closed contacts of switch KI 0|, and to condensers 0-433 and 0-434, opposite terminals of which are grounded. The ungrounded connection to the condensers and RI46 leads to the screen grids of V-l0| and I08, and to the plates of V-IB'I and VI08 by way of the center tap 4 and end connections 3 and 5 of T|02. The capacitance-resistance combination in the normally unoperated condition of switch Kl 0| and the inductive coils of the primary of T-l02, act as an effective power or noise filter.

In operation, the jack Jl0l is plugged into the hydrophone output. The signal passes through transformer T--I0l and is amplified in preamplifier V-l0l and V-I02. The operator selects the filter band by adjusting switch S--I 04 for optimum operation. The signal coming from the filter is amplified in amplifier V-I 03, V-l 06, V--|0| and V--l08 and fed to high impedance headphones through jack J-l03. If it is desirable to use low impedance headphones they are fed through jack J-I04 and transformer TI 02.

The rectifier 7 The operator now has the apparatus in operating condition and can proceed with the scanning, which he accomplished by orienting the hydrophone through 360. If a signal suggestive of the noise of a vessel is received, the filter bands may be readjusted to determine the predominant frequencies. A bearing of the signal may be secured by orienting the hydrophone until the signal strength is a maximum in the headphones and reading the bearing of the hydrophone. However, this method may be improved upon by turning on electron ray V-I04, and since the electron ray operates on the higher frequencies and the hydrophone is more sensitive to direction at the higher frequencies, the orienting can be made by the pattern of the electron ray more accurately than by ear.

Assuming that a vessel has been detected, it may be desirable to know Whether or not the said vessel is searching with echo ranging and detecting equipment. This can be determined by throwing detector switch S-l02 which renders audible in the headphones any supersonic frequencies present.

Hydrophones require magnetizing at intervals. This may be accomplished by connecting the magnetizer input J--l02 to the hydrophone input J--|0l and closing key Kl0l. When this is done the filter condensers in the push pull output circuit are discharged through the hydrophone for magnetizing purposes.

What is claimed is:

In an amplifier for underwater listening apparatus, a preamplifier stage receiving the input from the hydrophone and delivering the output to a selective band filter, a selective band filter, a final amplifier stage receiving the input from said filter and delivering the output to utilization apparatus, a noise filter in the output circuit of said final amplifier including normally charged condensers, means including a coil surrounding said hydrophone and connected through a switch to said condensers for supplying at will a magnetizing current for the hydrophone, an electron-ray device connected to the output circuit of said final amplifier for bearing determination purposes and a detector connected to the input circuit of the last stage of the final amplifier to render audible in the utilization apparatus any supersonic frequencies present.

FRANK P. HERRNFELD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,730,411 Dorsey' Oct. 8, 1929 2,063,944 Pierce Dec. 15, 1936 2,339,198 Smith Jan. 11, 1944 

